Many investments have been made to research and development of a technology called Internet of Things (IoT) for its practical use, in which various sensors are installed in equipment and information from these sensors is collected and analyzed to understand the operating status of the equipment for optimally managing a system that includes the equipment. In order to materialize communication with a large number of sensors installed in the equipment, the sensors include radio devices to form a wireless network with the radio devices. In such a wireless network, it is difficult to form a line-of-sight propagation path without any shielding object between a pair of radio devices communicating with each other, and thus the radio devices communicate in an over-the-horizen communication environment (i.e., over-the-horizen communication) using an electromagnetic wave reflected by the equipment which is generally an electromagnetic wave scatterer. The electromagnetic wave transmitted from a transmitter reaches a receiver via a plurality of different wireless propagation paths through single or multiple reflection(s).
Wireless communication using a conventional generic technique has a problem such that a plurality of wireless propagation paths formed between a transmitting point and a receiving point are concurrently used all at once, to cause the propagated information to have its quality degraded or to be altered to different information, if a failure occurs in a specific propagation path of the plurality of wireless propagation paths at a time of obtaining sensor information. In wireless communication, the wireless propagation paths as communication paths are open to the outside to cause a specific propagation path to be made to have a failure, and wireless communication using a conventional generic technique does not allow for identifying a specific wireless propagation path. This problem inhibits IoT technology from being adapted to infrastructure systems that provide lifelines and particularly require stability and safety of operation in a strict manner.
Japanese Patent Application Publication No. 2006-094215 provides a means for solving this problem. Japanese Patent Application Publication No. 2006-094215 discloses a technique of a wireless communication receiving device as described “[Problem] To increase receiving performance while reducing its throughput [Solution to the Problem] An estimation means 105 estimates the existence of a plurality of paths based on a received signal. A rake combining means 117 executes rake processing of transmission path estimation, phase amplitude correction, and despreading for each signal corresponding to each path, and combines a plurality of signals subjected to the rake processing. A frequency equalizing means 118 executes frequency equalization on a signal obtained by Fourier transforming the received signal, and despreads the processed signal. An operation selecting means 105 selects one of the rake combining means and the frequency equalizing means depending on the existence of the plurality of paths.” (see ABSTRACT)
As cited above, the technique disclosed in Japanese Patent Application Publication No. 2006-094215 uses the correlation value of the CDMA spreading code to detect the phase difference of the electromagnetic waves for identifying the difference in the path lengths of the plurality of wireless propagation paths formed between the transmitting and receiving points.